On the subsurface CFC maxima in the subtropical North Pacific thermocline and their relation to mode waters and oxygen maxima

Abstract

The spatial structure of the subsurface chlorofluorocarbon (CFC) concentration maxima in the subtropical North Pacific is examined using data from the Pacific Ocean sector of the World Ocean Circulation Experiment (WOCE) Hydrographic Program collected in the period 1991–1994. Comparison with potential vorticity (PV) sections reveals that the CFC maxima coincide with the PV minimum associated with Subtropical Mode Water (STMW) in the western Pacific as suggested by earlier observations [Warner et al., 1996]. In the eastern part of the central Pacific the CFC maxima appear to be shallower than the Central Mode Water (CMW) while they are deeper than the Eastern Subtropical Mode Water in the eastern Pacific. A subsurface oxygen maximum also coincides with STMW and the CFC maxima in the west, is located between the CFC maxima and CMW in the central Pacific, and is absent in the east. The collocation in the west is explained by the fact that STMW is the densest water to outcrop in the recirculation to the south of the Kuroshio and the Kuroshio Extension while denser water is not directly ventilated in this region. Data from two post‐WOCE cruises and results from a steady state age model indicate that the CFC maxima are transient and are deepening such that the collocation of the CFC maxima and STMW does not persist in the late 1990s. Model experiments based on a smooth age profile demonstrate that the deepening of the CFC maxima is closely related to changes in the atmospheric CFC growth rates.